Association of BMI
With Mortality

Many of the observational
epidemiologic studies of BMI and mortality have reported a 'U-' or 'J-shaped'
relationship between BMI and mortality (28). Mortality rates are elevated in
persons with low BMI (usually below 20) as well as in persons with high BMI (28, 31, 32).In some
studies, adjustment for factors that potentially confound the relationship
between BMI and mortality, such as smoking status and pre-existing illness,
tends to reduce the upturn in mortality rate at low BMIs (31), but in a meta-analysis the higher mortality
at low BMIs was not eliminated after adjustment for confounding factors (32). It is unclear whether the elevated
mortality observed at low BMIs is due to an artifact of incomplete control for
confounding factors (285), inadequate body
fat and/or inadequate body protein stores that result from unintentional weight
loss (286), or individual genetic factors.
Currently, there is no evidence that intentional weight gain in persons with
low BMIs will lead to a reduction in mortality.